In recent years, magnetic materials are applied to electromagnetic wave absorbers, magnetic inks and devices such as an inductance element, and their importance is increasing year after year. Those parts use the characteristics of a magnetic permeability real part (relative magnetic permeability real part) μ′ and a magnetic permeability imaginary part (relative magnetic permeability imaginary part) μ″ of a magnetic material in accordance with a purpose. For example, an inductance element uses high μ′ (and low μ″) and an electromagnetic wave absorber uses high μ″. Consequently, in the case of actually using the characteristics in a device, μ′ and μ″ have to be controlled in accordance with a frequency band used by the device. In recent years, the frequency bands used by devices are high, so that a technique of manufacturing a material capable of controlling μ′ and μ″ at high frequencies is in strong demand.
As the magnetic materials for an inductance element used at high frequencies of 1 MHz or higher, ferrite and amorphous alloys are mainly used. The magnetic materials do not have a loss (low μ″), have high μ′, and display excellent magnetic characteristics in the range of 1 MHz to 10 MHz. However, the magnetic permeability real part μ′ of the magnetic materials drops in a higher frequency range of 10 MHz or higher, and satisfactory characteristics are not always obtained.
Development of an inductance element using the thin film technique such as sputtering and plating is also actively performed, and it is confirmed that the inductance element displays excellent characteristics also in a high frequency band. However, large equipment is necessary for the thin film technique such as sputtering, and film thickness and the like has to be controlled precisely. Therefore, the method is not always sufficiently satisfactory from the viewpoints of cost and yield. The inductance element obtained by the thin film technique also has a problem that thermal stability for long time of magnetic characteristics at high temperature and high moisture is insufficient.
A magnetic material having high μ′ and low μ″ in a high frequency band is expected to be applied to a device of high frequency communication equipment such as an antenna device. A present portable communication terminal performs most of information propagations by transmitting/receiving electrical waves. The frequency band of electrical waves presently used is a high frequency band of 100 MHz or higher. Attention is therefore being paid to electronic parts and substrates useful in the high frequency band. In a portable mobile communication and a satellite communication, electrical waves in a high frequency band such as GHz band are used.
To handle the electrical waves in such a high frequency band, energy loss and transmission loss in an electronic part have to be small. For example, in an antenna indispensable for a portable communication terminal, a transmission loss occurs in a transmitting process. The transmission loss is unpreferable since electrical waves are consumed as thermal energy in an electronic part and a substrate and causes heat generation in the electronic part. As a result, electrical waves to be transmitted to the outside are cancelled each other out. Consequently, electrical waves stronger than necessary have to be transmitted, and there is a problem from the viewpoint of effective use of power. The more the antenna is miniaturized, the more the problem of the transmission losses becomes conspicuous.
In recent years, with increasing demands for smaller and lighter communication devices, electronic parts are becoming smaller and spaces are being reduced. Despite this, it is necessary for an antenna to assure some distance from an electronic part and a substrate in order to suppress transmission loss for the above-described reason. Consequently, an unnecessary space has to be provided, and a problem arises that it is difficult to reduce the space.
To address the problem, an antenna using dielectric ceramics is developed. By achieving miniaturization of an antenna, the space can be reduced. However, since the dielectric material has dielectric loss, the transmission loss becomes large, and transmission/reception sensitivity cannot be obtained. Under present condition, the antenna is used as an auxiliary antenna, and there is a limitation. The dielectric material tends to narrow the resonance frequency band of an antenna, so that it is unpreferable to use dielectric material for a wideband antenna.
As a method of miniaturizing an antenna and saving power, there is a method of performing transmission/reception by passing electrical waves, which are to arrive at electrical parts and a substrate of communication devices from the antenna, to an insulating substrate of high magnetic permeability (high μ′ and low μ″) without passing the electrical waves to the electrical parts and the substrate. The method is more preferable for the reason that miniaturization of the antenna and power saving can be realized and, simultaneously, the band of the resonance frequency of the antenna can be widened.
A normal high magnetic permeability material is a metal or alloy. Since the normal high magnetic permeability materials are metals, electrical resistance is low, and the antenna characteristic deteriorates. Consequently, the materials cannot be used. In the case of using the high magnetic permeability material for an antenna substrate, the high magnetic permeability material has to have high insulting property.
On the other hand, in the case of using the high magnetic permeability material of an insulating oxide typified by ferrite for an antenna substrate, deterioration in the antenna characteristics caused by low electrical resistance can be suppressed. However, at high frequencies of a few hundreds Hz, the frequencies are close to resonance frequency, a transmission loss due to resonance becomes conspicuous, and the high magnetic permeability material cannot be used.
In the case of using the high magnetic permeability material for an antenna substrate, the thickness of the material of 10 μm or more, preferably, 100 μm or more is necessary. Under the present set of circumstances, there is no insulating high magnetic permeability material with high permeability in a high frequency band, particularly, in a GHz band, and having a thickness of 10 μm or more, preferably, 100 μm or more. Consequently, as the material of the antenna substrate, an insulating high magnetic permeability material (high μ′ and low μ″) in which transmission loss is suppressed as much as possible and which can be used for electrical waves of high frequencies is demanded.
On the other hand, an electromagnetic absorber absorbs noise which occurs as the frequency of an electronic device becomes higher by using high μ″, thereby reducing inconveniences such as erroneous operation of the electronic device. Examples of the electronic device are a semiconductor device such as an IC chip and various communication devices. There are various electronic devices used in high frequency band from 1 MHz to a few GHz, further, tens GHz or higher.
Particularly, in recent years, there is a tendency that electronic devices used in the high frequency band of 1 GHz or higher increase. An electromagnetic wave absorber of an electronic device used in a high frequency band is conventionally manufactured by mixing ferrite particles, carbonyl iron particles, FeAlSi flakes, FeCrAl flakes, or the like with a resin as a binder. However, μ′ and μ″ of those materials are extremely low in a high frequency band of 1 GHz or higher, and satisfactory characteristics are not always obtained. A material combined by the mechanical alloying method or the like lacks thermal stability for long hours and has a problem that the yield is low.
JP-A 2006-97123 (KOKAI) discloses, as a magnetic material for use at high frequencies, a core-shell magnetic material in which metal particles are coated with an inorganic material in multiple layers.